1. Field of the Invention
The invention relates generally to polymer compositions having desired properties. More particularly, embodiments of the invention relate to closures composed of polyethylene resin compositions.
2. Background of the Invention
This invention relates generally to polymer molding compositions useful for molding applications, and especially for screw closure applications. For screw closures, such as soda bottle caps, typically a polypropylene (PP) polymer is used as the cap closure for the needed strength, with an inner liner comprised of soft ethylene/vinyl acetate (EVA), PVC, SEBS, TPV or butyl rubber etc., to provide an excellent seal. However, this two part construction is costly and users are always looking for ways to solve problems at lower costs. It would be easier and more convenient to use a one piece closure, without a liner. However, such a design is difficult, because the cap closure must be strong enough to withstand the closure needs and soft enough to provide an excellent seal while not contaminating the taste or odor of the bottle contents. Specifically, the cap closure must hold the pressure of the contents and have good stress crack resistance and impact strength, in addition to good removal torque and strip torque requirements with excellent color.
With the advent of polyethylene-terepthalate (PET) soda bottles, polypropylene (PP) bottle cap resins were developed to fill the need for user-friendly, tamper-proof and low-cost closures for said bottles. The closure is in the form of a cap that is fitted on to the bottle having a tear off tab or security ring at the base as a tamper-proof feature. While the current PP closures are cheap, they are too stiff by themselves to form a gas tight seal with the bottle. So an ethylene vinyl acetate (EVA) copolymer or other liner was affixed in a second operation to the inner side of the PP cap as the sealing layer. This provides a soft polymer seal to provide a gas tight closure. This closure or cap construction functions well, but it is costly to produce due to the two part construction. Thus, there is a continuing need for new polymer solutions to satisfy or exceed the technical needs of the closure and to do so cheaper. This applies to both pressurized and non pressurized containers.
In attempts to solve this closure problem, there are many examples of polymer compositions useful for molding applications, including blends of linear low density polyethylene (LLDPE) with high density polyethylene (HDPE). For example, U.S. Pat. No. 5,981,664, the disclosure of which is incorporated herein by reference, discloses resin compositions for molding comprising 40-65% of an ethylene polymer having a density from 0.948 g/cm3 to 0.964 g/cm3, a melt flow index from 6 to 20 g/10 minutes and a mean molar mass distribution, Mw/Mn, from 2 to 5 and 35-60% of an ethylene polymer having a density from 0.935 g/cm3 to 0.953 g/cm3, a melt flow index from 0.1 to 0.35 g/10 minutes and a mean molar mass distribution, Mw/Mn, from 6 to 20, wherein the final composition has a density from 0.948 g/cm3 to 0.957 g/cm3, a melt flow index from 1.0 to 2.0 g/10 minutes and a mean molar mass distribution, Mw/Mn, from 3 to 10, and wherein the difference in densities of the two components is from 0 to 0.029 g/cm3. Such compositions are said to be useful for screw closures.
However, there is still a need for polymer formulations that can be molded into closures having acceptable properties, such as no need for liners, acceptable taste and odor, and satisfactory stress crack resistance and impact strength.